The invention relates to an electric incandescent lamp, in particular an electric incandescent lamp having a lamp vessel, at least one filament which is arranged in the lamp vessel and comprises a filament element for generating radiation in the infrared region and in the visible region, and at least one filter, which is applied at least partially to the lamp vessel, reflects radiation in the infrared region and is transparent in the visible region, at least for selected wavelengths of radiation. Such an electric incandescent lamp is known from EP 0 588 541.
The invention further relates to a method for producing such an electric incandescent lamp.
The radiation emitted by an incandescent lamp is a function of three factors, specifically the filament temperature T, the spectral emittance xcex5 of the radiating surface, and the area A of the radiating surface (Stefan-Boltzmann law). In the case of incandescent lamps, the two first mentioned factors are bounded below by the melting temperature and the temperature- and wavelength-dependent spectral emittance xcex5 of the filament material. The radiating surface A of a helix is calculated in accordance with equation 1 as
A=xcfx80xc2x7Dxc2x7Lxe2x80x83xe2x80x83(1)
where D=wire diameter and L=effective wire length.
A typical value for A is circa 30 mm2 for a 12 V/50 W halogen incandescent lamp.
A disadvantageous effect is exerted on the efficiency by losses which are determined essentially by the power (circa 62%) converted into IR radiation, and by the end losses (circa 10%) and the fill-gas losses (circa 10%). In order significantly to reduce IR losses, coatings (IRC=InfraRed Coating) which reflect IR radiation have been developed for the bulbs of incandescent lamps, such as are also mentioned, for example, in EP 0 588 541. It is important in this regard that the arrangement of incandescent helix and coating reflecting IR radiation must be such that the reflected IR radiation is focussed onto the incandescent helix. The cause of an unfocussed reflection can, for example, be that the filament axis does not run parallel to the bulb axis, and the helix sag occurring over the lifetime of an incandescent lamp. In particular since the layer reflecting IR radiation is usually attached to the outside of the bulb, it is to be borne in mind in the case of ellipsoid bulbs that the outer contour of the bulb can deviate from the desired geometry. It is also to be taken into consideration that the probability of absorption decreases strongly in the case of multiple reflections.
The already mentioned EP 0 588 541 has therefore addressed the object of proposing an electric incandescent lamp in which the helix and the layer reflecting IR radiation are arranged relative to one another in an essentially unfocussed relationship, and yet satisfactory absorption of IR radiation is ensured. In order to achieve this object, EP 0 588 541 provides an incandescent filament which comprises coiled segments of tungsten wire which are connected to one another and are supported by segment bearings in between the segments in an essentially rectangular frame.
A disadvantage of this solution is, on the one hand, that the segments made from coiled tungsten wire cannot be packed tightly enough in order to ensure a high probability that the IR radiation is already led back to the incandescent filament after at most two reflections, since at a high packing density there is a risk of short circuits between individual coiled wire segments, for example owing to increases in size or vibrations. It is also to be considered that an arc can be formed, and that the helixes can break off at the connecting points to the bearing frame. A substantial disadvantage exists, in particular, in that the coiling of the tungsten wire leads to a so-called xe2x80x9cradiation blackeningxe2x80x9d. To be specific, because of the temperature-dependence of its spectral emission coefficient, pure tungsten, which is preferably applied as filament material, has a light yield which is higher at the same temperature by circa 40% than the black body. This gain in selectivity is lost in part upon coiling the wire.
It would be possible to counter a reduction in the radiation blackening by enlarging the pitch. However, this would contradict the requirement for compact filaments.
Furthermore, there is a disadvantage with the incandescent lamp according to the prior art of EP 0 588 541 in that only materials which permit coiling with regard to their brittleness come into consideration for the incandescent helixes.
It is therefore the object of the present invention to propose an incandescent lamp which permits the construction of compact filaments in conjunction with minimum radiation blackening. In addition, it is to reduce the risks of helix short circuits, the breaking off of the helix at its point of suspension and the formation of arcs, and to permit a high degree of absorption of IR radiation by the filament. Furthermore, it is to permit, for the filaments, the use of materials which are not suitable for coiling because of their material properties.
This object is achieved by providing the electric incandescent lamp of the generic type with a filament element which is of flat, in particular strip-shaped construction.
It is also an object of the present invention to propose a method for producing such an electric incandescent lamp. This object is achieved by means of a method having the steps in accordance with claim 31.
The production of the helix is completely eliminated by the flat construction of the filament element. The outlay on adjustment turns out to be exceptionally slight owing to the inherent adjustment of a filament, formed from one or more flat filament elements, with reference to the layer reflecting IR radiation. Particularly in the case of the production of elliptical bulbs, the requirements placed on the geometry of the bulb can be kept slight, as a result of which there is an appreciable reduction in the outlay on production here, as well. There is necessarily a substantial reduction in the rejection proportion owing to the inherent adjustment.
The flat filament element used in accordance with the invention has a substantially higher light yield at the same temperature than a coiled filament, since the radiation blackening mentioned at the beginning does not occur in the case of uncoiled flat filament elements.
In the case of the use of a single flat filament element to construct the filament, it is necessarily impossible for gaps to arise between individual segments, as a result of which it is possible to ensure given an appropriately wide construction of the filament element in relation to the inside diameter of the lamp vessel that IR radiation impinges again on the filament element after at most two reflections at the layer reflecting IR radiation.
Because of the design, winding short circuits and the formation of arcs as well as the breaking off of the helix at the point of suspension does not occur.
In the preferred embodiment mentioned, the filament element is constructed in one layer, for example from tungsten. In order to promote the emission in an envisaged direction, it is possible for there to be situated opposite the surface of the filament element situated opposite this direction a layer for reflecting radiation at least in the visible region, for example a reflecting layer.
The thickness of the filament element is preferably circa 5 to 50 xcexcm. The slight filament cross section resulting therefrom leads to a low heat dissipation, and therefore additionally reduces the end losses. Given a foil thickness of 10 xcexcm, there is, for example, an increase in the surface of the 50 W helix mentioned at the beginning to 270 mm2, that is to say by a factor of 8.5.
In further preferred embodiments, the filament element is constructed in a plurality of layers. This permits the use for the radiating layer of materials which, for example owing to their brittleness, would not come into consideration for producing helixes. In particular, it is possible to make use here of materials with a higher emission coefficient than tungsten for constructing the radiating layer arranged on a base layer. It is possible, as a result, to realize layer thicknesses in the xcexcm range which are required to avoid absorption losses in the case of transparent radiating layers. By treating the surface or by using special coating techniques, for example nano-technology, it is possible to increase the surface of the radiating layer by a multiple with respect to the surface of the base layer.
In the case of multi-layer construction of the filament element, the base layer or an additional layer applied to the rear side of the base layer can be produced from a material which has a lower emission coefficient for radiation in the infrared and/or visible regions than the radiating layer. If light is to exit from the incandescent lamp only in a specific direction, it is possible to arrange a layer for reflecting radiation at least in the visible region, in particular a reflecting layer, opposite the base layer or the additional layer. Owing to the splitting up into a base layer, which can be multi-layered, in turn, and a radiating layer, it is possible to produce the material for the base layer, independently of its emission coefficient, from a material which is optimum for producing films and conducting current and, on the other hand, to tune the light-emitting radiating layer to the special requirements of high emission or high absorption in a specific fashion. Since the filament element does not have to be coiled, there is even the possibility of using materials other than metallic ones which have a high selectivity and high emission in the visible spectral region such as, for example, special ceramics.
The filament can be composed of a plurality of filament elements which can, for example, be arranged next to one another or offset in height. In the latter case, it is particularly advantageous to select the width of the filament elements so as to produce overlapping.
Both an individual filament element and a plurality of filament elements can be dimensioned with regard to their overall width such that the latter is 25-100% of the inside diameter of the lamp vessel.
By interconnecting a plurality of filament elements, it is possible to achieve overall areas which permit the temperature of the filament elements to be lowered to values of circa 2000 K. This leads to a sharp reduction in the rate of evaporation, and thus to an improvement in length of lifetime. The red shift connected with the temperature drop can be compensated by a blue filter expediently fitted on the luminaire side.
Because of the high light yield, it is possible to operate using solar cells, storage batteries etc. Lighting engineering which protects the environment thereby becomes possible in areas which are not connected to the electric supply mains.
Owing to the fact that the filament is connected in a lamp vessel to a clamping device, for example a spring, which holds the filament or the filament elements clamped, sagging of the flat filament elements, for example through ageing or as a function of the spatial assembly of the lamp, is avoided. The current path in the lamp body correspondingly comprises a section which is of variable length and is preferably arranged parallel to the clamping device and can, for example, comprise a plurality of folded molybdenum strips arranged in parallel.
It has proved to be particularly advantageous to construct the layer reflecting IR radiation in the form of a plurality of filters which are arranged one behind another in the direction of propagation of radiation and are tuned to one another with respect to their wavelength-dependent reflection factors so as to produce a high total reflection factor for radiation in the infrared region. Mixed metallic and dielectric systems are preferably used for the coating.
Owing to the fact that a foil section, for example made from molybdenum foil, is provided in the current path in the region of a pinch of the lamp vessel, heating there by IR radiation is eliminated. The connections are therefore colder than in the case of known embodiments, and this leads to a further reduction in the end losses.
Depending on the application, the lamp vessel can be evacuated or filled with a fill-gas, the fill-gas advantageously containing at least one halogen.
Different variants come into consideration with regard to the arrangement of filament and lamp vessel: firstly, filament and lamp vessel can be of flat construction and arranged parallel to one another; however, they can also be of concentric construction. For example, the lamp vessel can be arranged concentrically around the filament. It is then particularly advantageous if the filament is arranged concentrically around a layer for reflecting radiation at least in the visible region, in particular a reflecting layer. The lamp vessel can in this case have a round, elliptical or rectangular cross section.
Further advantageous developments of the invention are defined in the subclaims.